The present invention relates to power devices, and in particular relates to minimizing reverse leakage current in such devices in wide bandgap semiconductor materials.
Power semiconductor devices operate at high voltage and thus in the presence of, or otherwise generate or experience, high electric fields. Such devices typically include, but are not necessarily limited to Schottky (rectifying) diodes, metal-oxide semiconductor field-effect transistors (MOSFETs); insulated gate bipolar transistors (IGBTs); PIN diodes; bipolar junction transistors (BJTs). For example (but not as a limitation), SiC-based power devices are advantageous for (switching) power supplies, motor control, power conditioning, hybrid vehicle technology, safety equipment, and power storage.
For electronic power devices, silicon carbide offers a number of physical, chemical and electronic advantages. Physically, the material is very hard and has an extremely high melting point, giving it robust physical characteristics. Chemically, silicon carbide is highly resistant to chemical attack and thus offers chemical stability as well as thermal stability. Perhaps most importantly, however, silicon carbide has excellent electronic properties, including high breakdown field, a relatively wide band gap (about 2.9 eV at room temperature for the 6H polytype), high saturated electron drift velocity, giving it significant advantages with respect to high power operation, high temperature operation, radiation hardness, and absorption and emission of high energy photons in the blue, violet, and ultraviolet regions of the spectrum.
As another wide bandgap material, gallium nitride offers similar advantages. It has a wide bandgap (about 3.4 eV at room temperature), high thermal conductivity, a high melting point, a low dielectric constant and a high breakdown voltage.
For power applications, silicon carbide's wide bandgap results in a high impact ionization energy. In turn, this allows SiC to experience relatively high electric fields without avalanche multiplication of ionized carriers. By way of comparison, silicon carbide's electric field capacity is about ten times as great as that of silicon.
Because the active regions of these devices experience or generate such high electric fields, the devices typically must include some sort of termination structure to lessen the effects of the field (“field crowding”) at the edge of the device. In common examples, the termination structure includes implanted regions in the silicon carbide adjacent the active region. Because the surface of the device must also be terminated, some sort of passivation structure is typically added to this surface. In most cases, the surface passivation structure can include a polymer (frequently polyimide) or a dielectric passivation such as silicon oxide, silicon nitride, or some combination of these, including non-stoichiometric oxides and non-stoichiometric nitrides (i.e., other than SiO2 and Si3N4).
Reverse leakage current in power diodes, particularly Schottky (rectifying) diodes, is a source of power loss that in turn reduces system efficiency and increases the operating temperature of a power device. Although several mechanisms can contribute to reverse leakage current, one source includes surface generation of carriers outside of the active region of the device. Although a proportionally small part of the device, the region of the diode between the termination structure and the edge of the physical die appears to contribute significantly to reverse bias leakage. Under certain conditions, this leakage mechanism appears to be dominated by surface generation. For example, in the presence of an oxide-semiconductor interface and a depletion condition, surface recombination-generation centers will generate a significant number of carriers that will in turn result in an increased leakage current.
As noted above, power diodes typically include some form of surface passivation or termination, but these typically fail to address the surface-generated leakage mechanism that tends to occur under depletion conditions. Similarly, the surface of a device (i.e., its overall size) could theoretically be expanded to proportionally minimize surface effects, but the resulting decrease in yield (fewer devices per wafer) and increased cost would be disproportionally unfavorable compared to the minimal affect on leakage current.
Accordingly, surface-generated carriers that are produced under depletion conditions represents a problem that exhibits itself as reverse leakage current.